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commit    93ed18720105ebacfd2a2bdeb55bf33feaf2ec80
Author:   Richard Guo <rguo@postgresql.org>
Date:     Sat Apr 11 07:38:47 2026 +0000
First released in: 17.10, 18.4

    Fix estimate_array_length error with set-operation array coercions
    
    When a nested set operation's output type doesn't match the parent's
    expected type, recurse_set_operations builds a projection target list
    using generate_setop_tlist with varno 0.  If the required type
    coercion involves an ArrayCoerceExpr, estimate_array_length could be
    called on such a Var, and would pass it to examine_variable, which
    errors in find_base_rel because varno 0 has no valid relation entry.
    
    Fix by skipping the statistics lookup for Vars with varno 0.
    
    Bug introduced by commit 9391f7152.  Back-patch to v17, where
    estimate_array_length was taught to use statistics.
    
    Reported-by: Justin Pryzby <pryzby@telsasoft.com>
    Author: Tender Wang <tndrwang@gmail.com>
    Reviewed-by: Richard Guo <guofenglinux@gmail.com>
    Discussion: https://postgr.es/m/adjW8rfPDkplC7lF@pryzbyj2023
    Backpatch-through: 17


 src/backend/utils/adt/selfuncs.c    | 12 ++++++++++++
 src/test/regress/expected/union.out | 17 +++++++++++++++++
 src/test/regress/sql/union.sql      |  4 ++++
 3 files changed, 33 insertions(+)

diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index d5468edcf31..2c1daefc0c0 100644
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -2165,6 +2165,18 @@ estimate_array_length(PlannerInfo *root, Node *arrayexpr)
 		AttStatsSlot sslot;
 		double		nelem = 0;
 
+		/*
+		 * Skip calling examine_variable for Var with varno 0, which has no
+		 * valid relation entry and would error in find_base_rel.  Such a Var
+		 * can appear when a nested set operation's output type doesn't match
+		 * the parent's expected type, because recurse_set_operations builds a
+		 * projection target list using generate_setop_tlist with varno 0, and
+		 * if the required type coercion involves an ArrayCoerceExpr, we can
+		 * be called on that Var.
+		 */
+		if (IsA(arrayexpr, Var) && ((Var *) arrayexpr)->varno == 0)
+			return 10;			/* default guess, should match scalararraysel */
+
 		examine_variable(root, arrayexpr, 0, &vardata);
 		if (HeapTupleIsValid(vardata.statsTuple))
 		{
diff --git a/src/test/regress/expected/union.out b/src/test/regress/expected/union.out
index 0fd0e1c38b3..882b075b9a1 100644
--- a/src/test/regress/expected/union.out
+++ b/src/test/regress/expected/union.out
@@ -1489,3 +1489,20 @@ on true limit 1;
                ->  Result
 (6 rows)
 
+-- Test handling of Vars with varno 0 in estimate_array_length
+explain (verbose, costs off)
+select null::int[] union all select null::int[] union all select null::bigint[];
+                 QUERY PLAN                  
+---------------------------------------------
+ Append
+   ->  Result
+         Output: (NULL::integer[])
+         ->  Append
+               ->  Result
+                     Output: NULL::integer[]
+               ->  Result
+                     Output: NULL::integer[]
+   ->  Result
+         Output: NULL::bigint[]
+(10 rows)
+
diff --git a/src/test/regress/sql/union.sql b/src/test/regress/sql/union.sql
index f8826514e42..c52ca012b5c 100644
--- a/src/test/regress/sql/union.sql
+++ b/src/test/regress/sql/union.sql
@@ -577,3 +577,7 @@ select * from tenk1 t1
 left join lateral
   (select t1.tenthous from tenk2 t2 union all (values(1)))
 on true limit 1;
+
+-- Test handling of Vars with varno 0 in estimate_array_length
+explain (verbose, costs off)
+select null::int[] union all select null::int[] union all select null::bigint[];


[parent: c05c3baf169f]